Against Regis is Machamp the best counter?

I agree, having a 6 Machamp squad handles all the Regis fairly well. Kyogre is better against Regirock and Moltres/Entei against Registeel/Regice, so put them first in your team if you have them powered up.

That said, I wouldn't prepare too heavily for Regis as raid bosses right now. Latis will last for two more months and we don't even know if Regis will become raid bosses after that, they could just get released through the research stamp system.

Machamp (with C/DP) is the best counter against a lot of things, and a serviceable counter against most of the remaining things, so if you don't already have a Machamp army of six with C/DP, get to work on acquiring them.

Unless it's against Pokemon that resist Fighting-types (or, in some cases, against Pokemon that can deal SE damage against Machamp), Machamp is never a bad counter option. He has a widespread use in many scenarios, so having a team of six Machamps is always beneficial.